Separator for rechargeable lithium battery and rechargeable lithium battery including the same

ABSTRACT

A separator for a rechargeable lithium battery includes a substrate; and a coating layer positioned on at least one side of the substrate, wherein a thickness ratio of the coating layer relative to the total thickness of the substrate and the coating layer ranges from about 5% to about 50%, and a loading level of the coating layer ranges from about 1.4 g/m 2  to about 9.8 g/m 2 , and a rechargeable lithium battery including the same is provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2014-0163793, filed in the Korean IntellectualProperty Office on Nov. 21, 2014, the entire content of which isincorporated herein by reference.

BACKGROUND

1. Field

A separator for a rechargeable lithium battery and a rechargeablelithium battery including the same are disclosed.

2. Description of the Related Art

A rechargeable lithium battery includes a positive electrode, a negativeelectrode and a separator interposed between the positive and negativeelectrodes. The separator includes micropores, and the micropores play arole of electrically insulating the positive and negative electrodes aswell as provide a movement passage for lithium ions. In addition, theseparator shuts down the battery when the battery temperature goes overa set or predetermined temperature and thus, plays a role of preventingthe battery from being overheated.

Due to the recent demand for high power/large capacity batteries forelectric vehicles, the separator is required to be thinner and lighter,and simultaneously, have excellent thermal shape stability to produce ahigh-capacity battery.

For this purpose, a separator obtained by coating a binder resin and aceramic particle on a porous substrate is mainly used (utilized).However, the separator is contracted (e.g., shrank) when the battery isoverheated, and thus, may hardly meet the thermal stability requirement.

SUMMARY

An aspect according to one embodiment of the present invention isdirected towards a separator for a rechargeable lithium battery havingimproved thermal safety.

Another aspect according to one embodiment of the present invention isdirected towards a rechargeable lithium battery including the separatorfor a rechargeable lithium battery.

According to one embodiment of the present invention, a separator for arechargeable lithium battery includes a substrate; and a coating layeron at least one side of the substrate, wherein a thickness ratio of thethickness of the coating layer to the total thickness of the substrateand the coating layer may be from about 5% to about 50%, and a loadinglevel of the coating layer may be from about 1.4 g/m² to about 9.8 g/m².

The thickness ratio of the thickness of the coating layer to the totalthickness of the substrate and the coating layer may be from about 15%to about 45%.

The loading level of the coating layer may be from about 2.5 g/m² toabout 5.0 g/m².

The coating layer may have a thickness of about 1 μm to about 7 μm.

The coating layer may include two or more kinds of inorganic particleshaving different particle diameters from each other.

The inorganic particles may have an average particle diameter (D50) ofabout 0.1 μm to about 5 μm.

The inorganic particles may include a first particle and a secondparticle having a smaller average particle diameter than the firstparticle, the first particle may be included in a larger amount than thesecond particle, and in one embodiment, a weight ratio of the firstparticle and the second particle may be from about 9.5:0.5 to about 6:4.

The inorganic particles may include SiO₂, Al₂O₃, Al(OH)₃, AlO(OH), TiO₂,BaTiO₂, ZnO₂, Mg(OH)₂, MgO, Ti(OH)₄, aluminum nitride (AlN), siliconcarbide (SiC), boron nitride (BoN), clay, a glass powder, or acombination thereof.

The coating layer may further include a binder, the binder may include apolyacrylic acid-based compound, and the polyacrylic acid-based compoundmay include poly(meth)acrylic acid, a poly(meth)acrylate salt, or acombination thereof.

The inorganic particles may be included in an amount of about 70 toabout 96 wt % based on the total amount of the inorganic particles andthe binder.

The coating layer may further include a dispersing agent, and thedispersing agent may include an acryl-based compound.

The dispersing agent may be included in an amount of about 0.1 to about5 parts by weight based on 100 parts by weight of the inorganicparticles.

According to another embodiment of the present invention, a rechargeablelithium battery includes the separator.

Other example embodiments are included in the following detaileddescription.

The separator may realize a rechargeable lithium battery having improvedthermal safety.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a rechargeable lithium batteryaccording to one embodiment.

FIG. 2 is a graph showing cycle-life characteristics of the rechargeablelithium battery cells according to Examples 1 to 3 and ComparativeExamples 1 and 2 as a function of the cycle repetition.

DETAILED DESCRIPTION

Hereinafter, embodiments are described in more detail. However, theseembodiments are exemplary, and this disclosure is not limited thereto.Expressions such as “at least one of,” when preceding a list ofelements, modify the entire list of elements and do not modify theindividual elements of the list. Further, the use of “may” whendescribing embodiments of the present invention refers to “one or moreembodiments of the present invention.” Also, the term “exemplary” isintended to refer to an example or illustration. It will be understoodthat when an element or layer is referred to as being “on”, “connectedto”, “coupled to”, or “adjacent to” another element or layer, it can bedirectly on, connected to, coupled to, or adjacent to the other elementor layer, or one or more intervening elements or layers may be present.In contrast, when an element or layer is referred to as being “directlyon,” “directly connected to”, “directly coupled to”, or “immediatelyadjacent to” another element or layer, there are no intervening elementsor layers present. As used herein, the term “substantially,” “about,”and similar terms are used as terms of approximation and not as terms ofdegree, and are intended to account for the inherent deviations inmeasured or calculated values that would be recognized by those ofordinary skill in the art. Also, any numerical range recited herein isintended to include all sub-ranges of the same numerical precisionsubsumed within the recited range. For example, a range of “1.0 to 10.0”is intended to include all subranges between (and including) the recitedminimum value of 1.0 and the recited maximum value of 10.0, that is,having a minimum value equal to or greater than 1.0 and a maximum valueequal to or less than 10.0, such as, for example, 2.4 to 7.6. Anymaximum numerical limitation recited herein is intended to include alllower numerical limitations subsumed therein and any minimum numericallimitation recited in this specification is intended to include allhigher numerical limitations subsumed therein. Accordingly, Applicantreserves the right to amend this specification, including the claims, toexpressly recite any sub-range subsumed within the ranges expresslyrecited herein. All such ranges are intended to be inherently describedin this specification such that amending to expressly recite any suchsubranges would comply with the requirements of 35 U.S.C. §1 12, firstparagraph, and 35 U.S.C. §132(a).

Hereinafter, a separator for a rechargeable lithium battery according toone embodiment is described.

The separator for a rechargeable lithium battery according to oneembodiment separates a negative electrode from a positive electrode andprovides transfer passages for lithium ions, and includes a substrateand a coating layer positioned on at least one side of the substrate.

The substrate may have a porous structure including pores. Lithium ionsare transferred through the pores. The substrate may use (utilize)polyolefin (such as polyethylene, polypropylene, or the like),polyester, polytetrafluoroethylene (PTFE), a glass fiber, or acombination thereof. The substrate may be a non-woven fabric or a wovenfabric. The substrate may have a single layer structure or a multilayerstructure. For example, the substrate may be a polyethylene singlelayer, a polypropylene single layer, a polyethylene/polypropylene doublelayer, a polypropylene/polyethylene/polypropylene triple layer, apolyethylene/polypropylene/polyethylene triple layer, or the like. Thesubstrate may have a thickness of about 1 μm to about 40 μm, and forexample, about 1 μm to about 30 μm, about 1 μm to about 20 μm, about 5μm to about 15 μm, or about 5 μm to about 10 μm. When the substrate hasa thickness within the above ranges, a short circuit between thepositive and negative electrodes may be prevented, while the internalresistance of the battery is not increased.

The coating layer may include inorganic particles. The inorganicparticles may include at least two kinds of inorganic particles havingdifferent particle diameters from each other. When at least two kinds ofinorganic particles having different particle diameters from each otherare used (utilized) to form the coating layer, a short circuit betweenthe positive and negative electrodes may be suppressed by reducing orpreventing thermal contraction of the separator, and battery performancemay be improved by reducing or minimizing resistance of lithium ions.

The inorganic particles may have an average particle diameter (D50) ofabout 0.1 μm to about 5 μm, for example, about 0.2 μm to about 3 μm.Within the average particle diameter range, at least two inorganicparticles (e.g., two kinds of inorganic particles, each including aplurality of inorganic particles) having a different particle diameterfrom each other may be mixed. In one embodiment, the inorganic particlesmay include a first particle (e.g., a first kind of particles includinga plurality of particles having a first average particle diameter (D50))and a second particle (e.g., a second kind of particles including aplurality of particles having a second average particle diameter (D50))having a smaller particle diameter (e.g., average particle diameter)than the first particle. When the inorganic particles having relativelylarge and small sizes are mixed, a separator having improved heatresistance may be secured. For example, the large and small particlesare distinguished with reference to a particle size of about 0.5 μmwithin an average particle diameter (D50) ranging from about 0.1 μm toabout 5 μm. That is, the relatively larger particle, i.e., the firstparticle, may have an average particle diameter (D50) of greater than orequal to about 0.5 μm and less than or equal to about 5 μm, for example,greater than or equal to about 0.6 μm and less than or equal to about 3μm; and the relatively smaller particle, i.e., the second particle, mayhave an average particle diameter (D50) of greater than or equal toabout 0.1 μm and less than about 0.5 μm, for example, greater than orequal to about 0.2 μm and less than about 0.4 μm.

In one embodiment, the first particle may be mixed in a larger amountthan the second particle. In one embodiment, the first particle and thesecond particle may be mixed in a weight ratio of about 9.5:0.5 to about6:4, for example, about 9:1 to about 7:3. When the inorganic particleshaving larger and smaller particle diameters are mixed within the aboveweight ratio ranges, stability and reliability may be secured byreducing or minimizing the moisture content of a separator, andsimultaneously, thermal stability may be improved by reducing orpreventing contraction and rupture of a separator and thus, reducing orpreventing ignition and explosion of the separator at a hightemperature.

The inorganic particle may include SiO₂, Al₂O₃, Al(OH)₃, AlO(OH), TiO₂,BaTiO₂, ZnO₂, Mg(OH)₂, MgO, Ti(OH)₄, aluminum nitride (AlN), siliconcarbide (SiC), boron nitride (BoN), clay, a glass powder, or acombination thereof, and for example, AlO(OH) may be used (utilized).

In addition, the moisture content of the separator may be adjusted bycontrolling the thickness of the coating layer relative to the substrateand the loading level of the coating layer, and thus, thermal stabilitymay be improved.

The thickness of the coating layer relative to the substrate and theloading level of the coating layer may be adjusted in a method offorming the coating layer, for example, by adjusting the coating layercomposition or the like. In one embodiment, at least two kinds ofinorganic particles having different particle diameters from each otherand specifically, large and small particles are used (utilized) withinthe weight ratio range described above to adjust the thickness of thecoating layer relative to the substrate and the loading level of thecoating layer. As a result, the moisture content of the separator may bereduced or minimized, and simultaneously, the shape of the separator maybe maintained without being melted down, thereby realizing excellentcycle-life characteristics of a battery.

In one embodiment, a thickness ratio of the thickness of the coatinglayer relative to the total thickness of the substrate and the coatinglayer (i.e., the sum of the thickness of the substrate and that of thecoating layer) may be about 5% to about 50%, for example, about 15% toabout 45%, or about 30% to about 35%. When the coating layer has athickness ratio relative to the substrate within the above ranges, themoisture content of a separator may be reduced or minimized, andsimultaneously, excellent thermal stability may be secured.

The coating layer may have a thickness of about 1 μm to about 7 μm, forexample, about 2 μm to about 6 μm, or about 4 μm to about 5 μm. When thecoating layer has a thickness within the above ranges, a separatorhaving the coating layer does not only have excellent adherence to anelectrode plate but also reduced or minimized thermal contraction andthus, may realize a rechargeable lithium battery having excellentthermal safety.

In addition, a loading level of the coating layer may be about 1.4 g/m²to about 9.8 g/m², for example, about 2.0 g/m² to about 7 g/m², about2.5 g/m² to about 5.0 g/m², or about 3.0 g/m² to about 3.5 g/m². Whenthe loading level of the coating layer is within the above ranges, themoisture content of a separator may be reduced or minimized, andsimultaneously (also), excellent thermal stability may be secured. Theloading level refers to the weight of the coating layer per unit area ofthe substrate.

The coating layer may further include a binder. When the binder is used(utilized) with the above inorganic particles, a separator may befurther reduced or prevented from thermal contraction, and its adherenceto an electrode plate may be improved.

The binder may include a polyacrylic acid-based compound. Thepolyacrylic acid-based compound may include poly(meth)acrylic acid, apoly(meth)acrylate salt, or a combination thereof. Examples of thepoly(meth)acrylic acid may be polyacrylic acid, polymethacrylic acid,and a combination thereof. Examples of the poly(meth)acrylate salt maybe a salt compound of polyacrylic acid, polymethacrylic acid, and acombination thereof. Herein, the poly(meth)acrylate salt may be a saltcompound including an alkali metal, an alkaline-earth metal, ammonium,an amine salt, and a combination thereof. Examples of thepoly(meth)acrylate salt may be a sodium polyacrylate, a sodiumpolymethacrylate, a magnesium polyacrylate, a magnesiumpolymethacrylate, an ammonium polyacrylate, an ammoniumpolymethacrylate, and the like, but are not limited thereto.

The polyacrylic acid-based compound may have a viscosity of about 500cps to about 10,000 cps, for example, about 3,000 cps to about 6,000cps. When the polyacrylic acid-based compound has a viscosity within theabove ranges, heat resistance of a separator is improved, and thus, itsexcellent thermal stability may be secured. The viscosity may bemeasured by using (utilizing) a reference solvent of a mineral oil(KS1000 & 5000) as a standard solution for calibrating a viscometer.

The binder may further include a styrene-butadiene rubber (SBR),carboxylmethyl cellulose (CMC), ethylene vinylacetate (EVA),hydroxyethyl cellulose (HEC), polyvinyl alcohol (PVA), polyvinylbutyral(PVB), an ethylene-acrylic acid copolymer, acrylonitrile, vinyl acetatederivative, polyethylene glycol, an acryl-based rubber, or a combinationthereof, besides the polyacrylic acid-based compound.

The inorganic particles may be included in an amount of about 70 toabout 96 wt %, for example, about 80 to about 96 wt %, about 90 to about96 wt %, or about 93 to about 95 wt %, based on the total amount of theinorganic particles and the binder. When the inorganic particles areused (utilized) within the above ranges, the moisture content of aseparator may be reduced or minimized, and simultaneously, thermalcontraction of the separator at a high temperature is reduced orprevented, thus obtaining excellent thermal stability.

The coating layer may further include a dispersing agent to increasedispersion of the inorganic particles into water.

The dispersing agent may use (utilize) an acryl-based compound. Herein,the acryl-based compound may be different from the polyacrylicacid-based compound used (utilized) as a binder above.

The dispersing agent may be included in an amount of about 0.1 to about5 parts by weight, for example, about 0.1 to about 1 part by weight,based on 100 parts by weight of the inorganic particles. When thedispersing agent is used (utilized) within the above ranges, theinorganic particles are uniformly dispersed in the coating layer, andthus, a separator having excellent thermal stability may be secured.

The coating layer is formed by coating a composition including theinorganic particles, the binder, the dispersing agent, and deionizedwater on at least one side of the substrate and drying it.

The coating composition may be coated on the substrate using (utilizing)a dip coating method, a die coating method, a roll coating method, acomma coating method, or the like, without being limited thereto.

The drying may include drying using (utilizing) warm air, hot air, orlow humid air, or vacuum-drying, but the present invention is notlimited thereto.

Hereinafter, a rechargeable lithium battery including the separator isdescribed referring to FIG. 1.

FIG. 1 is a schematic view showing a rechargeable lithium batteryaccording to one embodiment.

Referring to FIG. 1, a rechargeable lithium battery 100 according to oneembodiment includes an electrode assembly including a positive electrode114, a negative electrode 112 facing the positive electrode 114, aseparator 113 interposed between the negative electrode 112 and thepositive electrode 114, an electrolyte solution impregnating thepositive electrode 114, the negative electrode 112, and the separator113, a battery case 120 housing the electrode assembly, and a sealingmember 140 sealing the battery case 120.

The separator 113 is the same as described above.

The positive electrode 114 includes a current collector and a positiveactive material layer formed on the current collector.

The current collector may use (utilize) aluminum, but is not limitedthereto.

The positive active material layer includes a positive active material.

The positive active material may be a compound (lithiated intercalationcompound) being capable of intercalating and deintercallating lithium,and in one embodiment, a lithium metal oxide.

The lithium metal oxide may, in one embodiment, include lithium and atleast one metal selected from cobalt, manganese, nickel and aluminum. Inone embodiment, compounds represented by one of the following chemicalformulae may be used (utilized).

Li_(a)A_(1-b)X_(b)D₂ (0.90≦a≦1.8, 0≦b≦0.5);Li_(a)A_(1-b)X_(b)O_(2-c)D_(c) (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05);Li_(a)E_(1-b)X_(b)O_(2-c)D_(c) (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05);Li_(a)E_(2-b)X_(b)O_(4-c)D_(c) (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05);Li_(a)Ni_(1-b-c)Co_(b)X_(c)D_(a) (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05, 0<α≦2);Li_(a)Ni_(1-b-c)Co_(b)X_(c)O_(2-α)T_(α) (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05,0<α<2); Li_(a)Ni_(1-b-c)Co_(b)X_(c)O_(2-a)T₂ (0.90≦a≦1.8, 0≦b≦0.5,0≦c≦0.05, 0<α<2); Li_(a)Ni_(1-b-c)Mn_(b)X_(c)D_(a) (0.90≦a≦1.8, 0≦b≦0.5,0≦c≦0.05, 0<a≦2); Li_(a)Ni_(1-b-c)Mn_(b)X_(c)O_(2-α)T_(α) (0.90≦a≦1.8,0≦b≦0.5, 0≦c≦0.05, 0<α<2); Li_(a)Ni_(1-b-c)Mn_(b)X_(c)O_(2-α)O₂(0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05, 0<α<2); Li_(a)Ni_(b)E_(c)G_(d)O₂(0.90≦a≦1.8, 0≦b≦0.9, 0≦c≦0.5, 0.001≦d≦0.1);Li_(a)Ni_(b)Co_(c)Mn_(d)G_(e)O₂ (0.90≦a≦1.8, 0≦b≦0.9, 0≦c≦0.5, 0≦d≦0.5,0.001≦e≦0.1); Li_(a)NiG_(b)O₂ (0.90≦a≦1.8, 0.001≦b≦0.1); Li_(a)CoG_(b)O₂(0.90≦a≦1.8, 0.001≦b≦0.1); Li_(a)Mn_(1-b)G_(b)O₂ (0.90≦a≦1.8,0.001≦b≦0.1); Li_(a)Mn₂G_(b)O₄ (0.90≦a≦1.8, 0.001≦b≦0.1);Li_(a)Mn_(1-g)G_(g)PO₄ (0.90≦a≦1.8, 0≦g≦0.5); QO₂; QS₂; LiQS₂; V₂O₅;LiV₂O₅; LiZO₂; LiNiVO₄; Li_((3-f))O₂(PO₄)₃(0≦f≦2); Li_((3-f))Fe₂(PO₄)₃(0≦f≦2); and LiFePO₄

In the chemical formulae, A is selected from Ni, Co, Mn, and acombination thereof; X is selected from AI, Ni, Co, Mn, Cr, Fe, Mg, Sr,V, a rare earth element, and a combination thereof; D is selected fromO, F, S, P, and a combination thereof; E is selected from Co, Mn, and acombination thereof; T is selected from F, S, P, and a combinationthereof; G is selected from Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, and acombination thereof; Q is selected from Ti, Mo, Mn, and a combinationthereof; Z is selected from Cr, V, Fe, Sc, Y, and a combination thereof;and J is selected from V, Cr, Mn, Co, Ni, Cu, and a combination thereof.

The lithium metal oxide may be, in one embodiment, a lithium nickelcobalt manganese oxide, a lithium nickel cobalt aluminum oxide, or acombination thereof, and for example, a mixture of the lithium nickelcobalt manganese oxide and the lithium nickel cobalt aluminum oxide maybe used (utilized).

Besides the positive active material, the positive active material layerincludes a binder and a conductive material.

The binder improves binding properties of the positive active materialparticles to each other and to a current collector. Examples of thebinder include polyvinyl alcohol, carboxylmethyl cellulose,hydroxypropyl cellulose, diacetyl cellulose, polyvinylchloride,carboxylated polyvinylchloride, polyvinylfluoride, an ethyleneoxide-containing polymer, polyvinylpyrrolidone, polyurethane,polytetrafluoroethylene, polyvinylidene fluoride, polyethylene,polypropylene, a styrene-butadiene rubber, an acrylatedstyrene-butadiene rubber, an epoxy resin, nylon, and the like, but arenot limited thereto.

The conductive material provides an electrode with conductivity. Anysuitable electrically conductive material may be used (utilized) as aconductive material unless it causes a chemical change. Examples of theconductive material include a carbon-based material (such as naturalgraphite, artificial graphite, carbon black, acetylene black, ketjenblack, carbon fiber, or the like); a metal-based material (such as ametal powder, a metal fiber, or the like of copper, nickel, aluminum,silver, or the like); a conductive polymer (such as a polyphenylenederivative or the like); and a mixture thereof.

The negative electrode 112 includes a current collector and a negativeactive material layer disposed on the current collector.

The current collector may be a copper foil, but is not limited thereto.

The negative active material layer includes a negative active material,a binder, and optionally, a conductive material.

The negative active material may include a material that reversiblyintercalates/deintercalates lithium ions, a lithium metal, a lithiummetal alloy, a material capable of doping and dedoping lithium, or atransition metal oxide.

The material that can reversibly intercalate/deintercalate lithium ionsmay include a carbon material. The carbon material may be anygenerally-used (utilized) carbon-based negative active material in alithium ion rechargeable battery. Examples of the carbon materialinclude crystalline carbon, amorphous carbon, and mixtures thereof. Thecrystalline carbon may be non-shaped (e.g., irregularly shaped), orsheet, flake, spherical, or fiber shaped natural graphite or artificialgraphite. The amorphous carbon may be a soft carbon (carbon fired at alow temperature), a hard carbon, a mesophase pitch carbonizationproduct, fired coke, or the like.

Examples of the lithium metal alloy include lithium and a metal selectedfrom Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge,Al, and Sn.

The material being capable of doping/dedoping lithium may include Si,SiO_(x) (0<x<2), a Si—C composite, a Si-Q alloy (wherein Q is an alkalimetal, an alkaline-earth metal, Group 13 to Group 16 elements, atransition metal, a rare earth element, or a combination thereof, butnot Si), Sn, SnO₂, a Sn—C composite, Sn—R (wherein R is an alkali metal,an alkaline-earth metal, Group 13 to Group 16 elements, a transitionmetal, a rare earth element, or a combination thereof, but not Sn), orthe like. At least one of these materials may be mixed with SiO₂. Theelements Q and R may be Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V,Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd,Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, TI, Ge, P, As, Sb, Bi, S, Se,Te, Po, or a combination thereof.

The transition metal oxide may include vanadium oxide, lithium vanadiumoxide, or the like.

The binder improves binding properties of the negative active materialparticles with one another and with a current collector. Examples of thebinder include polyvinylalcohol, carboxylmethylcellulose,hydroxypropylcellulose, polyvinylchloride, carboxylatedpolyvinylchloride, polyvinylfluoride, an ethylene oxide-containingpolymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene,polyvinylidene fluoride, polyethylene, polypropylene, astyrene-butadiene rubber, an acrylated styrene-butadiene rubber, anepoxy resin, nylon, and the like, but are not limited thereto.

The conductive material is included to improve electrode conductivity.Any suitable electrically conductive material may be used (utilized) asa conductive material unless it causes a chemical change. Examples ofthe conductive material include a carbon-based material (such as naturalgraphite, artificial graphite, carbon black, acetylene black, ketjenblack, a carbon fiber, or the like); a metal-based material (such as ametal powder, a metal fiber, or the like of copper, nickel, aluminum,silver, or the like); a conductive polymer (such as a polyphenylenederivative); and a mixture thereof.

The negative electrode may be manufactured by mixing the negative activematerial, the binder and the conductive material in a solvent to preparea negative active material composition, and coating the negative activematerial composition on the current collector. Herein, the solvent maybe N-methylpyrrolidone, or the like, and an aqueous solvent such aswater or the like may be used (utilized) according to the kind of thebinder, but is not limited thereto.

The electrolyte solution includes a non-aqueous organic solvent and alithium salt.

The non-aqueous organic solvent serves as a medium for transmitting ionstaking part in the electrochemical reaction of a battery. Thenon-aqueous organic solvent may be selected from a carbonate-based,ester-based, ether-based, ketone-based, alcohol-based and aproticsolvent.

The carbonate-based solvent may be, for example, dimethyl carbonate(DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropylcarbonate (MPC), ethylpropyl carbonate (EPC), ethylmethyl carbonate(EMC), ethylene carbonate (EC), propylene carbonate (PC), butylenecarbonate (BC), or the like.

In one embodiment, when the carbonate-based solvent is prepared bymixing a cyclic carbonate and a linear carbonate, a solvent having a lowviscosity while having an increased dielectric constant may be obtained.The cyclic carbonate and the linear carbonate may be mixed together inthe volume ratio of about 1:1 to 1:9.

The ester-based solvent may include, for example, methyl acetate, ethylacetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethylpropionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone,caprolactone, or the like. The ether-based solvent may include, forexample, dibutylether, tetraglyme, diglyme, dimethoxyethane,2-methyltetrahydrofuran, tetrahydrofuran, or the like. The ketone-basedsolvent may include cyclohexanone, or the like. The alcohol-basedsolvent may include ethanol, isopropyl alcohol, or the like.

The non-aqueous organic solvent may be used (utilized) singularly or ina mixture. When the organic solvent is used (utilized) in a mixture, themixture ratio can be controlled in accordance with a desirable batteryperformance.

The non-aqueous electrolyte solution may further include anovercharge-inhibiting additive such as ethylene carbonate,pyrocarbonate, or the like.

The lithium salt dissolved in the non-aqueous organic solvent supplieslithium ions in the battery, and operates a basic operation of arechargeable lithium battery and improves lithium ion transportationbetween the positive and negative electrodes.

Examples of the lithium salt may include one selected from LiPF₆, LiBF₄,LiSbF₆, LiAsF₆, LiN(SO₃C₂F₆)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₂, LiAlCl₄,LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) (wherein x and y are naturalnumbers), LiCl, LiI, LiB(C₂O₄)₂ (lithium bis(oxalato)borate; LiBOB), anda combination thereof.

The lithium salt may be used (utilized) at a concentration ranging fromabout 0.1 M to about 2.0 M. When the lithium salt is included at theabove concentration range, an electrolyte solution may have excellentperformance and lithium ion mobility due to appropriate conductivity andviscosity of an electrolyte solution.

Hereinafter, the embodiments are illustrated in more detail withreference to examples. However, these examples are exemplary, and thepresent disclosure is not limited thereto. Furthermore, what is notdescribed in this disclosure may be sufficiently understood by those whohave knowledge in this field and will not be illustrated here.

Manufacture of Separator Example 1

An inorganic solution was prepared by mixing 45.5 wt % of a mixture ofAlO(OH) having an average particle diameter (D50) of 1.5 μm and AlO(OH)having an average particle diameter (D50) of 0.3 μm in a weight ratio of9:1 as the inorganic particles, 0.5 wt % of a dispersing agentcontaining 40% of a nonvolatile component (CERASPERSE 5468CF, SAN NOPCOLtd.), and 54 wt % of distilled water with a beadmill. In addition, abinder solution was prepared by mixing 4.9 wt % of polyvinyl alcohol(PVA217, KURARAY Inc.) and 95.1 wt % of distilled water. Then, 61.7 wt %of the inorganic solution and 38.3 wt % of the binder solution weremixed to prepare a slurry.

The slurry was coated on one side of a 11 μm-thick single polyethylenefilm to form a 3 μm-thick coating layer, thereby complete themanufacturing of a separator.

Example 2

A separator was manufactured according to the same method as Example 1except for preparing an inorganic solution by using (utilizing) amixture of AlO(OH) having an average particle diameter (D50) of 1.5 μmand AlO(OH) having an average particle diameter (D50) of 0.3 μm in aweight ratio of 8:2 as the inorganic particles.

Example 3

A separator was manufactured according to the same method as Example 1except for preparing an inorganic solution by using (utilizing) amixture of AlO(OH) having an average particle diameter (D50) of 1.5 μmand AlO(OH) having an average particle diameter (D50) of 0.3 μm in aweight ratio of 7:3 as the inorganic particles.

Comparative Example 1

A separator was manufactured according to the same method as Example 1except for preparing an inorganic solution by mixing 45.5 wt % ofAlO(OH) having an average particle diameter (D50) of 0.3 μm, 0.5 wt % ofa dispersing agent, and 54 wt % of distilled water with a beadmill.

Comparative Example 2

A separator was manufactured according to the same method as Example 1except for preparing an inorganic solution by mixing 45.5 wt % ofAlO(OH) having an average particle diameter (D50) of 1.5 μm, 0.5 wt % ofa dispersing agent, and 54 wt % of distilled water with a beadmill.

(Manufacture of Rechargeable Lithium Battery Cell)

LiCoO₂, polyvinylidene fluoride and carbon black in a weight ratio of97:1.5:1.5 were added to an N-methylpyrrolidone (NMP) solvent to preparea slurry. The slurry was coated on an aluminum (Al) thin film and then,dried and compressed, thereby complete the manufacturing of a positiveelectrode.

On the other hand, graphite, styrene-butadiene rubber and carbon blackin a weight ratio of 98:1:1 were added to an N-methylpyrrolidone (NMP)solvent to prepare a slurry. The slurry was coated on a copper foil andthen, dried and compressed, thereby complete the manufacturing of anegative electrode.

An electrolyte solution was prepared by mixing ethylene carbonate (EC),ethylmethyl carbonate (EMC) and diethyl carbonate (DEC) in a volumeratio of 3:5:2, and adding 1 M LiPF₆ to the mixed solvent.

The positive electrode, the negative electrode and the electrolytesolution were used (utilized) with each separator according to Examples1 to 3 and Comparative Examples 1 and 2, thereby complete themanufacturing of a rechargeable lithium battery cell.

Evaluation 1: Moisture Content of Separator

A moisture content included in the coating layer of each separatoraccording to Examples 1 to 3 and Comparative Examples 1 and 2 wasmeasured, and the result is provided in the following Table 1.

The moisture content was measured by using (utilizing) Karl Fischer 860KF thermoprep and 831 KF coulometer. A separation membrane was put in avial and fixed on the moisture meter (thermoprep), and its moisturecontent was measured while the separator was maintained at 150° C. for600 seconds and then, the result is converted into ppm. However, themoisture content was measured even after 600 seconds until it came downto less than or equal to 3 ug/min.

Evaluation 2: Air Permeability of Separator

Air permeability of the separators according to Examples 1 to 3 andComparative Examples 1 and 2 was measured utilizing the followingmethod, and the results are provided in the following Table 1.

Each separator was cut into a size of 6 cm×6 cm, and its airpermeability was measured by using (utilizing) a Gurley densometer. Theair permeability was obtained by injecting 100 cc of air with a set orpredetermined pressure and measuring how long it took for the air tocompletely pass pores of the separator.

Evaluation 3: Rupture Test of Separator

A rupture test about the separators according to Examples 1 to 3 andComparative Examples 1 and 2 was performed, and the results are providedin the following Table 1.

A 5 cm×5 cm of each separator was fixed in a paper frame with an imidetape, then, put in an oven, heated up to 230° C., and maintained for 10minutes, and then, the shape of the separator was examined. Herein, whenthe separator maintained its shape, it was classified as PASS, whilewhen the separator was contracted or broken, it was classified as FAIL,and data in the following Table shows the number of FAIL samples/thenumber of tested samples.

TABLE 1 Coating Loading Substrate layer level Moisture Air thicknessthickness Thickness of coating content permeability Rupture (μm) (μm)ratio (%) layer (g/m²) (ppm) (sec/0.1 L) test Ex. 1 11 3 21.4 3.4 320141 0/5 Ex. 2 11 3 21.4 3.3 371 138 0/5 Ex. 3 11 3 21.4 3.3 662 127 0/5Comp. Ex. 1 13.5 0.5 3.57 0.8 1211 110 5/5 Comp. Ex. 2 6 8 57.1 10.2 290148 2/5

In Table 1, a thickness ratio (%) was obtained as a percentage of thethickness of a coating layer relative to the total thickness of thecoating layer and a substrate.

Referring to Table 1, the separators including a coating layer formed bymixing two or more kinds of inorganic particles having differentparticle diameters from each other in an appropriate ratio according toExamples 1 to 3 showed a thickness ratio of the coating layer relativeto a substrate and the loading level of the coating layer within anappropriate range compared with the separators according to ComparativeExamples 1 and 2. Herein, the separators of Examples 1 to 3 showed asmall moisture content and excellent air permeability compared with theseparators of Comparative Examples 1 and 2 and also, were neithercontracted nor broken at high temperature and thus, may realize arechargeable lithium battery having excellent thermal stability.

Evaluation 4: Cycle-life Characteristics of Rechargeable Lithium BatteryCell

Rechargeable lithium battery cells of Examples 1 to 3 and ComparativeExamples 1 and 2 were charged and discharged under the following chargeand discharge condition, and their cycle-life characteristics wereevaluated, and the results are provided in FIG. 2.

The charge was performed in a CC-CV mode at 4.2 V, 0.5 C, and 0.05 C ofa current cut-off, and the discharge was performed in a CC-mode at 3.0V, and 0.5 C, and the charge and discharge were repeated 100 times.

FIG. 2 is a graph showing cycle-life characteristics of the rechargeablelithium battery cells of Examples 1 to 3 and Comparative Examples 1 and2 as a function of the repetitive cycle.

Referring to FIG. 2, Examples 1 to 3 (each using (utilizing) a mixtureof two or more kinds of inorganic particles having different particlediameters from each other with an appropriate ratio to form a coatinglayer for a separator and thus, having a thickness ratio of the coatinglayer relative to a substrate and a loading level of the coating layerwithin an appropriate range) showed excellent cycle-life characteristicscompared with Comparative Examples 1 and 2 (each using (utilizing) onekind of inorganic particles).

While this disclosure has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims, and equivalents thereof.

DESCRIPTION OF SYMBOLS

-   100: rechargeable lithium battery-   112: negative electrode-   113: separator-   114: positive electrode-   120: battery case-   140: sealing member

What is claimed is:
 1. A separator for a rechargeable lithium battery comprising a substrate; and a coating layer on at least one side of the substrate, wherein a thickness ratio of a thickness of the coating layer to a total thickness of the substrate and the coating layer is from about 5% to about 50%, and a loading level of the coating layer is from about 1.4 g/m² to about 9.8 g/m².
 2. The separator of claim 1, wherein the thickness ratio of the thickness of the coating layer to the total thickness of the substrate and the coating layer is from about 15% to about 45%.
 3. The separator of claim 1, wherein the loading level of the coating layer is from about 2.5 g/m² to about 5.0 g/m².
 4. The separator of claim 1, wherein the coating layer has a thickness of about 1 μm to about 7 μm.
 5. The separator of claim 1, wherein the coating layer comprises two or more kinds of inorganic particles having different particle diameters from each other.
 6. The separator of claim 5, wherein the inorganic particles have an average particle diameter (D50) of about 0.1 μm to about 5 μm.
 7. The separator of claim 5, wherein the inorganic particles comprise a first particle and a second particle having a smaller particle diameter than the first particle, and the first particle is included in a larger amount than the second particle.
 8. The separator of claim 7, wherein a weight ratio of the first particle and the second particle is from about 9.5:0.5 to about 6:4.
 9. The separator of claim 5, wherein the inorganic particles comprise SiO₂, Al₂O₃, Al(OH)₃, AlO(OH), TiO₂, BaTiO₂, ZnO₂, Mg(OH)₂, MgO, Ti(OH)₄, aluminum nitride (AlN), silicon carbide (SiC), boron nitride (BoN), clay, a glass powder, or a combination thereof.
 10. The separator of claim 5, wherein the coating layer further comprises a binder.
 11. The separator of claim 10, wherein the binder comprises a polyacrylic acid-based compound, and the polyacrylic acid-based compound comprises poly(meth)acrylic acid, a poly(meth)acrylate salt, or a combination thereof.
 12. The separator of claim 10, wherein the inorganic particles are included in an amount of about 70 to about 96 wt % based on a total amount of the inorganic particles and the binder.
 13. The separator claim 5, wherein the coating layer further comprises a dispersing agent, and the dispersing agent comprises an acryl-based compound.
 14. The separator of claim 13, wherein the dispersing agent is included in an amount of about 0.1 to about 5 parts by weight based on 100 parts by weight of the inorganic particles.
 15. A rechargeable lithium battery comprising the separator of claim
 1. 